This application is based on application No. 98-44507 filed in the Korea Industrial Property Office on Oct. 23, 1998, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a non-aqueous electrolyte for electrochemical systems, and more particularly, to a non-aqueous electrolyte for electrochemical systems capable of generating an electromotive force by an action of lithium ion, in conjunction with at least one of the electrodes comprising of carbons with graphite structure.
(b) Description of the Related Art
As technologies in the electronics have been tremendously improved, the use of various portable electronics including the notebook computers and mobile communication devices has become widespread in recent years. As a power source of driving these portable electronics, the demand for secondary batteries has been increased. Although the conventional lead batteries and nickel-cadmium batteries have excellent performances, these aqueous solution type batteries are not satisfactory in weight and energy density. Therefore, non-aqueous electrolyte secondary batteries that can exhibit high voltage and high energy density have been desired.
Non-aqueous electrolyte batteries using lithium or material that can store lithium as their electrode active material have been investigated due to the advantage that lithium is the lightest and also most electropositive metal that can potentially provide high voltage and lightweight energy sources. Lithium secondary batteries are conventionally constituted of a pair of electrodes that can store and emit lithium reversibly; a porous insulating membrane interposed between the electrodes; and a non-aqueous electrolyte comprising a lithium salt and a mixture of aprotic solvents.
By convention, the negative electrode defined as the anode is the more electropositive electrode that is oxidized upon discharge. The positive electrode defined as the cathode is the more electronegative electrode that is reduced upon discharge. Typically lithium ion is released from the negative electrode upon discharge and stored by the positive electrode. For example, U.S. Pat. No. 4,035,555 discloses a battery comprising niobium tetraselenide as positive electrode material, lithium metal as negative electrode material and non-aqueous electrolyte with propylene carbonate (PC) as the solvent. Upon discharge, lithium ion released from the lithium anode is transferred to the cathode to form lithium intercalated niobium tetraselenide. The potential use of various metal oxide and metal chalcogenide as a positive electrode material was reviewed more comprehensively in D. W. Murphy and P. A. Christian, Science, Vol. 205, 1979, page 4407.
As a negative electrode material, lithium metal has been used as a lithium source. Although the lithium metal can provide a high energy density negative electrode, it presents a cycle life problem and a safety concern because of poor lithium plating efficiency and intrinsically high chemical reactivity. Upon repeated cycling, surface of the lithium electrode becomes porous due to dendritic plating of lithium. The formation of such structure with large surface area is undesirable because it reacts violently with moisture and air. Furthermore the battery is assembled as charged state when lithium metal is used as a negative electrode, which also requires special attention to prevent internal or external short circuit in the production of such batteries, especially when the high voltage material is used for the positive electrode. Lithium alloys also present similar problems in the handling of the negative electrode and of the resulting batteries, even though the cycle life and safety can be improved. Furthermore lithium alloys have limited ductility and their uses are severely limited to batteries that do not require much curvature of the electrode.
On the other hand, U.S. Pat. No. 4,423,125 discloses a non-aqueous lithium secondary battery, which comprises a lithiated graphite instead of lithium metal or lithium alloys for the negative electrode. Since this battery uses graphite as a host of lithium storage capable of reversibly intercalating and de-intercalating lithium ions, it exhibits good cycle life characteristics. However, in order to operate in conjunction with a positive electrode that does not contain extractable lithium, the graphite electrode should be prelithiated to be electrochemically active. Such pre-lithiation may present problems because the pre-lithiated graphite is highly reactive to moisture, and because it adds an extra step of intercalating lithium uniformly to the graphite without forming metallic lithium.
As a new cathode active material that contains extractable lithium, U.S. Pat. No. 4,302,518 discloses a lithium-containing transition metal oxide having a layer structure. A secondary lithium battery can be produced by combining this material with a negative electrode comprising of a more electropositive material that is capable of reversibly intercalating and de-intercalating lithium ion. For example, a graphite electrode can be used as such a negative electrode. When graphite or other carbon material is used for an anode, it exhibits good cycle life and improved safety characteristics. Such battery is assembled as inactive state, and thereby requires to be charged in order to be electrochemically active.
One drawback of the carbon anode is the poor Coulombic efficiency at the first cycle, lowering the battery capacity. Therefore, when LiCoO2 is initially used for the positive electrode, the x in LiCoO2 after the first charge-discharge cycle becomes much less than the initial value. Such irreversible capacity arises from the reaction of electrolyte on the carbon surface, and accompanies a consumption of lithium ion that is initially released from the positive electrode. The amount of lithium capacity consumed in the initial charge cycle depends on the composition of non-aqueous electrolyte and the type of anode active material.
Meanwhile, the carbon material is normally divided into amorphous carbons having only partial and short-range order and high-crystalline graphitic carbons with a well-defined long-range crystalline order. The amorphous carbons are advantageous in terms of the interfacial stability toward wide range of electrolytic solvents including conventional cyclic esters such as PC and gamma butyrolactone (xcex3BL). In terms of the energy density, the amorphous carbons are not as satisfactory as the high-crystalline graphitic carbons, due to its low true density. However, if the high-crystalline graphitic carbons are used for the negative electrode in a non-aqueous electrolyte secondary battery containing PC or xcex3-BL for its electrolyte, the battery is not fully charged, and practical battery performances are not obtained. Such failure is known to arise from the reaction of these electrolytes on the surface of highly crystalline graphitic carbons, as reported for examples in A. N. Dey and B. P. Sullivan, J. Electrochem. Soc., Vol. 117, 1970, page 222 and Fujimoto et al, J. Power Sources, Vol. 63, 1996, page 127.
Thus, to fully utilize the high energy density of the high crystalline graphitic carbons, it is critical to use an appropriate electrolyte. Conventional electrolytes used in nonaqueous electrolyte lithium batteries consist of a mixture of organic solvents and a lithium salt. The requirements for the organic solvent are a capability of dissolving a large amount of lithium salt (or a high dielectric constant) and a capability of conducting the dissolved lithium ion over the operating temperature range of the battery. Examples of a solvent with a high dielectric constant include cyclic esters such as PC, xcex3-BL, ethylene carbonate (EC), and butylene carbonate (BC, conventionally referring to 1,2-butylene carbonate or in IUPAC name 4-ethyl-1,3-dioxolan-2-one). EC has a large dielectric constant but it cannot conduct lithium ion at ambient temperature because it is solid below 36xc2x0 C. Many other chemicals with high dielectric constant such as various small molecular anhydrides and 2-oxazolidinone tend to solidify easily at relatively high temperature. Furthermore, solvents with a large dielectric constant solvate lithium ion so strongly that it is co-intercalated into the graphite layers along with the lithium ion, when lithium ion is first intercalated into the graphitic carbons. Such co-intercalation leads to a large irreversibility in the first cycle and, particularly for PC, xcex3-BL, and BC, it completely prohibits the graphitic carbon from reversibly intercalating lithium ions.
As an example of a solvent mixture that can be used at ambient temperature to reversibly intercalate lithium into graphite after the first cycle, an ECIPC mixture is reported for example in R. Fong et al, J. Electrochem. Soc., Vol. 139, 1990, page 2009. However, such a mixture of EC and PC exhibits a large initial irreversible capacity loss. Hence it is practically not satisfactory as an electrolyte for non-aqueous lithium secondary batteries. It is further reported in Z. X. Shu et al, J. Electrochem. Soc., Vol. 140, 1993, page 922 that a large amount of gas is generated as a result of the irreversible reaction of the electrolyte on graphite surface. Similarly, the electrolytes containing xcex3-BL or BC are not compatible with graphitic carbon anode and thus cannot provide sufficient battery performances.
Meanwhile electrolyte mixtures consisting of a solvent with high dielectric constant and a solvent with low viscosity have been commonly used in non-aqueous electrochemical systems including capacitors and primary lithium batteries. improved lithium ion conductivity of such mixture is disclosed for example in U.S. Pat. No. 4,957,833 for the mixture of cyclic carbonate and linear carbonate. In particular, in several patents (U.S. Pat. Nos. 5,521,027 and 5,639,575), the mixture comprising EC and linear carbonate is disclosed for lithium secondary batteries consisting of high crystalline graphitic carbons as the negative electrode.
On the other hand, the electrolytes based on the mixture of EC and linear carbonates present a problem for high rate discharge at low temperature, because of high melting temperature of EC. Improved performance at low temperature is disclosed for example in U.S. Pat. No. 5,472,809 for electrolytes comprised of EC, PC and DMC and in U.S. Pat. No. 5,643,695 for mixtures of EC, PC, and linear carbonate, or EC, BC, and linear carbonate. However, due to the reactivity of PC and BC toward high crystalline graphitic carbons, it has a disadvantage that the structure of carbons that can be used for the negative electrode is severely limited to the ones with large surface area.
In view of the above described status of the art, an object of the present invention is to provide a non-aqueous electrolyte containing a solvent with high dielectric constant, where the electrolyte exhibits high conductivity at low temperature and small irreversibility toward a wide range of graphitic carbon anodes.
It is an object of the present invention to provide a non-aqueous electrolyte for use in electrochemical systems capable of generating an electromotive force by an action of lithium ion.
It is another object of the present invention to provide a non-aqueous electrolyte for use in electrochemical systems capable of generating an electromotive force by an intercalation of lithium ion, in conjunction with at least one of the electrodes comprising of carbons with graphite structure.
These and other objects may be achieved by a non-aqueous electrolyte for electrochemical systems including trans-4,5-dialkyl-1,3-dioxolan-2-one, where two alkyl group are independently having 1 to 2 carbons; and a lithium salt.
The non-aqueous electrolyte lithium secondary battery includes a carbon capable of intercalating and de-intercalating lithium ion for an anode; a lithium containing material that can reversibly intercalate and de-intercalate lithium ion for a cathode; and a non-aqueous electrolyte including trans-4,5-dialkyl-1,3-dioxolan-2-one and a lithium salt.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following description of the preferred embodiments, and accompanying drawings.